Begun and ended?
When the 24th hour leap started it didn't passed the 1 Day border.
The first year anno domini(oh!) was year 0.
Your first year to live on, was year 0 and you usually celebrate the passed first year as your first birthday.
2021 is the 21th century ...
Informaticians start counting at zero, but this confuses the "normal" people. :8~)
